Earlier this week we had a couple of warm days and a lot of melting. The sound—a steady drip and slap—overwhelmed even the enthusiastic songs of birds around the neighborhood. Water streamed from rooftops and soaked the trunks of trees. It flowed down driveways in sheets and converged on the street. Heading off to pick up my son after school, I walked between rivulets that slipped under piles of snow along the edges of the lane. Where winter ice had broken the asphalt, murky pools formed. But the pull of gravity was clear. Meltwater escaped through every crack, rejoining the flow and slipping westward toward the catch basin at the lowest point in our neighborhood watershed.

As I stood waiting for the bus, I unfocused my camera and took a picture of a spot where where tiny ripples swirled over the rough asphalt. The result is a patchwork of reflected light. I like the picture, but it leads me to something bigger. Water is in connection with the land as well as the light. What does that mean? Anything that lies on the street—leaves, dust, road salt, fertilizer, pest waste, and more—becomes part of the flow. Consequently, how we treat the land (and the air) affects the quality and quantity of water. It's a simple equation, but one we often forget.

Today is World Water Day. The United Nations cites increasing urbanization—the growth of population in cities—as a major influence on water resources globally. You'll use water many times today. When you do, stop to ponder this life-giving resource. Below are a few of the many available resources to help you learn how to protect it.

This morning I took a series of photographs as a downy woodpecker and blue jay fed outside the kitchen window. I noticed that both birds spent some "down time" on their respective feeders, appearing to rest for a bit between bouts of feeding. When I later downloaded the pictures, I found several shots showing each bird with its eyes partially or completely "closed."

The birds weren't actually napping. I was lucky enough to get photographs of their third eyelids—what biologists refer to as the nictating membrane. The term is drawn from the Latin word nictare ("to blink"). Birds have paired eyelids, as we do. These close vertically (from the top and bottom) when the bird sleeps. The nictating membrane is a separate structure located between the eyelids and the cornea. It usually remains hidden at the inner corner of the eye. The nictating membrane has two primary purposes: to clean and moisten the surface of the eye, and to protect it from injury.

The blue-gray nictating membrane can be seen across the anterior portion of this jay's eye.

A bird's sail-shaped nictating membrane is firmly affixed by ligaments at two points (the top of the eye and the side closest to the bill). The third attachment is movable, allowing it to sweep sideways over the cornea in a motion something like that of a windshield wiper. As I observed, in a quiet moment the bird may flick the nictating membrane open and shut a few times as a sort of preening action. The membrane is also closed when the bird needs protection from environmental hazards.

This structure shows a lot of adaptive variation. Raptors have a nearly transparent nictating membrane, allowing them to see while flying but also protecting the eye from injury caused by twigs, branches, or struggling prey. Imagine the benefits of such protection for fast-flying or pelagic (sea-faring) birds, which otherwise face the drying effects of wind and abrasion from small airborne particles. Woodpeckers and nuthatches have unusually thickened, opaque nictating membranes that protect their eyes from flying wood chips.

Normally, the nictating membrane is folded up and hidden, as seen on this downy woodpecker. The two outer lids (upper and lower) appear as a scaly ring around the eye.

The same bird, moments later, with the thick nictating membrane closed over its eye.

Nuthatches often cache seeds by wedging them under loose segments of tree bark. When the bird returns at a later date, rather than extracting the whole seed it chips away at the shell to reach the meat. The membrane protects the eye from dust and debris created while the bird feeds.

Like raptors, diving birds have a transparent nictating membrane. I'm very nearsighted and always appreciate the little bit of visual enhancement provided by goggles when I swim. I wondered if nictating membranes similarly improve the vision of birds swimming under water. Researchers looking at this question found that the membrane lies very close to the eye and has a curvature almost identical to that of the cornea (i.e., it is not faceted like my goggles). As a result, there is no significant refraction of light and no noticeable improvement to a bird's vision.

Nictating membranes are common in every vertebrate group except mammals, suggesting that the structure evolved in fishes and was lost much later by some mammals. Monotremes (the platypus and echidna, most "primitive" among the living mammals) have nictating membranes, as do marsupials. A few groups of placental mammals retain them, in particular those that are aquatic—seals and sea lions, manatees and dugongs, and beavers. For polar bears, nictating membranes serve the additional role of protecting against UV light that is so strong in the polar environment.

A tiny member of the loris family, found in West Africa, is the only primate with fully functional nictating membranes: able to clean the eye and move freely across it. But this structure has not been completely lost in other primates. Take a look in the mirror. See the pink, crescent-shaped blob at the inner corner of your eye? That is a nictating membrane—or what's left of it. Opthalmologists call this the plica semilunaris. It's often said to be vestigial, a scientific term for structures that have been retained through evolutionary time but lost their function. That's not an entirely accurate description in this case. Although the plica semilunaris has no ligamenture that permits movement, it still helps to clean the eye by producing fatty secretions to which pollen, dust, and other particles stick. These waste materials glom up and weep out. Nictare—blink, blink—and your eyes are cleaner. No goggles required.

From around the state I hear reports of the first migratory birds: sandhill cranes, hooded mergansers, and red-winged blackbirds. The numbers and diversity of birds-on-the-move will steadily increase through April and May, as longer days and warmer temperatures renew the availability of critical food sources. Stepping outside at night during the peak of migration, you can hear the sounds of their passing—distant contact calls, rustling wings, and a subtle wind that seems to carry spring behind it.

It's still cold enough to ruddy my cheeks and leave my gloved hands numb on a walk at Minnehaha Creek. But spring is emerging in subtle ways. Crows were chasing each other in loops over the treetops, and I could hear water swashing beneath the creek's surface layer of ice.

The burbling of chickadees makes a nice soundtrack as I head outside to shovel leftover snow from the deck and sidewalks. A group of three downy woodpeckers is not so happy with my timing. I am scolded after every aborted flight they make between the sugar maple and suet feeder: "Pik-pik-pik! You're too close!"